Our laboratory has had a long-standing interest in the biology of alloengraftment and development of methods to overcome resistance to engraftment of allogeneic stem cells. We hypothesize that the induction of stable mixed chimerism in sensitized nonmyeloablated recipients will be more readily achieved by manipulation of the immune rather than the hematopoietic system. We will utilize our expertise in conditioning regimens to design simulations in mice that are nontoxic and are directly extrapolatable to the clinic and immune strategies to prevent graft rejection and GVHD. Preclinical expts will be performed in well-established mouse models. Our specific aims are: Aim 1: To develop novel nontoxic conditioning regimens using pharmacological agents that are myelotoxic or lymphotoxic, an irradiation-based approach that is selectively lymphotoxic and keratinocyte growth factor to reduce regimen- related toxicity; Aim 2: To develop novel donor hematopoietic strategies using flt3 ligand alone or with G-CSF to increase donor HSC without increasing GVHD; Aim 3: To determine which immune-based approaches will facilitate alloengraftment without GVHD by using photochemical treatment of the donor lymphocytes, pharmacological agents that affect lymphokine production or responses, antibodies that affect T cell signalling or delayed donor lymphocytes infusion to treat low level chimeras; Aim 4: To determine whether our murine protocol can be applied to the treatment of hemoglobinopathies in mice and to ensure engraftment without toxicity when our proposed sickle cell anemia patient trial in matched sibling donors is applied to a setting of presentized recipients of unrelated donor grafts Reagents chosen for analysis are in clinical trials in humans or are soon to be studied in clinical trials in BMT patients. At the conclusion of our study, we will have gained the necessary experience and generated the required data to rapidly translate our studies into the clinic.